The present invention relates to removing contaminants from a surface of a substrate. The present invention is especially advantageous for cleaning semiconductor substrates, for example, silicon wafers after chemical processing. In particular, the present invention relates to applying a fluid to the surface to be cleaned, lowering the temperature of the fluid to form a solid layer of it and to entrap contaminants therein, and applying energy to separate the solid layer and the surface from each other.
In the fabrication of semiconductor devices, semiconductor substrates such as silicon wafers are typically subjected to a number of wet processing steps. Following wet processing, it is critical to dry these wafers leaving them free of chemical and particulate impurities. Typically drying is accomplished by physical removal of water from the wafer surface in centrifugal dryers, by displacement of water with a miscible and evaporative solvent (e.g. isopropyl alcohol) or by modification of the surface tension gradient (Marangoni effect). However, both of these techniques suffer from the problems of removal or elimination of contaminants and drying stains.
Contamination is always a concern in wafer processing, drying stains have been shown to be a deposition of colloidal silica on the surface of the wafers. Therefore, drying stains, in fact, are little more than very, very small and low-level particle-based residual which needs to agglomerate in large numbers in well defined regions of the wafer in order to be xe2x80x98seenxe2x80x99 or observed with either automated inspection systems or optical microscopes.
Drying stains occur because: (1) the equilibrium between dissolved silicates and colloidal silica is a strong function of pH, and (2) the media used for drying cause the water being removed from the surface to turn acidic when in contact with the drying medium, leading to the conversion of soluble silicates into silica, which can be deposited back onto the wafer surface.
Silicon compounds are present in the water (1) as a result of previous wafer processing where the substrate material (silicon) has been dissolved, (2) as an incoming impurity in the deionized (DI) water used to process the wafers or (3) as a result of hydrolysis of the wafer surface by the reactions:
Si0+2H2Oxe2x86x92SiO2+2H2 
or 
Si0+H2O+2OHxe2x88x92xe2x86x92SiO3=+2H2 
In the presence of an acid, soluble silicates can be converted to insoluble silica by the reaction:
SiO3=+2H+SiO2+H2O 
Centrifugal drying techniques are carried out in an atmospheric environment where a certain amount of CO2 is present. In water CO2 hydrolyses to carbonic acid. Other drying techniques use alcohols such as 2-propanol. Alcohols are also known to be weak acids, which can drive the precipitation of silicates on the wafer surface.
Accordingly, it would be desirable to provide an improved cleaning technique.
The present invention provides for an improved process for removing contaminants from a surface of a substrate. The process of the present invention makes it possible to clean and dry surfaces, including hydrophobic wafer surfaces, without forming watermarks, which reduce the yield.
More particularly, the present invention relates to a method of removing contaminants from the surface of a substrate. The method comprises applying a fluid to the surface to be cleaned; lowering the temperature of the fluid so as to form a solid layer of the fluid over the surface and to entrap contaminants within the layer; and applying energy such as sonic energy to the layer and/or the surface under such conditions as to result in separating the layer including the contaminants from the surface.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.